It appears this actually is documented, but you need to do some digging...
https://www.mercurial-scm.org/wiki/ChangeSetID
https://www.mercurial-scm.org/wiki/Nodeid
So the special nodeid you're referring to is the 'nullid'.
2 digits may not be adequate to identify the nullid as such (as it may be ambiguous if other hashes start with 2 zeros), so you may be better off specifying 4 0's or more.
Eg: hg diff -r 00:<hash of initial add changeset>
has resulted in the abort: 00changelog.i@00: ambiguous identifier!
error.